Coating the surface of a resin molded product employed for automobiles, electrical appliances and architectural members, has been widely performed for the purposes of adding a value such as fanciness or of increasing the weather resistance to prolong the service life of the product. As such a method of coating, a spray coating is commonly used. However, in recent years, strong interests are attracted to environmental problems, and from viewpoints that emission of harmful organic materials into the atmosphere from painting factories are becoming to be severely restricted and protection of employees' health should be considered to be more important, development of a technique instead of spray coating processes is urgently requested.
Under these circumstances, an in-mold coating process draws an attention, which is such a process that after injecting a coating agent between the surface of a resin molded product formed in a mold and a cavity surface of the mold, the coating agent is cured to obtain a integrally molded product which is a resin molded product having a coated film on its surface. The in-mold coating method is expected to remarkably contribute to reduce the cost by omitting the coating process.
Such in-mold coating process is mainly employed for improving the surface of molded products of thermosetting resin material such as SMC (Sheet Molding Compound) and BMC (Bulk Molding Compound). However, it has not been widely employed yet in the process of injection molding of a thermoplastic resin. One of the major reasons is as follows. Namely, when the thermosetting coating agent is cured in the mold by utilizing temperatures of the mold surface and the molded product surface, a high temperature of the mold and a long curing time are required to sufficiently cure the coating agent. However, the temperature of the mold is commonly set to a low temperature in a case of a thermoplastic resin material.
Such in-mold coating process in injection molding of a thermoplastic resin, is disclosed, for example, in JP-A-5-301251 and JP-A-5-318527.
Further, in JP-A-5-301251, the temperatures of a thermoplastic resin and the surface of the mold are set to be higher than the curing temperature of the coating agent.
However, the inventors of the present application have found that the appearance of the surface of a coating agent is influenced significantly by the nature of flowing of the coating agent when it flows in a mold, and that indisturbance of the flow front of the material is an important condition. Under the high mold temperature capable of sufficiently curing the coating agent as described above, the coating agent tends to be partially gelled or the viscosity of the coating agent tends to be increased during the flowing, which makes the flowing of the coating agent uneven and causes wrinkles on the surface of the coating agent or defects of the coating film such as mottles. Such a phenomenon is remarkable in a case of a coating agent containing an aluminum pigment or a pearl pigment, since the flow of the coating agent becomes uneven in the mold due to gelation or an increase of the viscosity and orientation of a flake pigment such as the aluminum pigment or the pearl pigment is disturbed, whereby generation of mottles or weldlines becomes remarkable. Therefore, until the coating agent covers the entire surface of the molded product to be coated, it is necessary to maintain sufficient flowability by preventing the gelation or the increase of the viscosity. However, for this purpose, it is necessary to either lower the temperature of the mold or to slow down the gelation of the coating agent. This means that the curing reaction is depressed and the curing time becomes extremely long, and in some cases, the curing enough to exhibit a sufficient performance could not be achieved, and therefore, such courses are undesirable.
Further, JP-A-5-318527 discloses a process of curing a thermosetting coating agent by heating a mold by e.g. a high-frequency induction heating to cure the coating agent after the coating agent is injected into the mold. However, in this process, since the mold is heated and cooled in the forming cycle, there are disadvantages of energy loss and a long forming cycle.
The present invention has been made under the above circumstances. It is an object of the present invention to provide a process for producing an in-mold coated molded product wherein after a forming material of thermoplastic resin is molded in a mold, a coating agent is coated on the surface of the molded product in the same mold whereby the molded product with a coating, which is free from wrinkles, cracks, mottles and weldlines in its cured coating film and has a high quality can be assured.